I have android application with version 1.0 installed on my nexses device which was developed by me, now i have done some modification in it and i signed the application with the same key which i signed the older version app. and finally i give it the version name 2.0. when i install this application on my nexses device it do not update the older application but it uninstall the older one 1.0 and replace it with new one 2.0

My question is that how to update application older one with new one without uninstalling older application?Plz do help me in that